From the text, page 144: "This figure [2] is estimated using Vital Statistics mortality records from 1997–2003." From personal correspondence, the rates were constructed using pooled data from death certificates that list months and years of birth and death (numerators) and interpolated Census figures on populations at risk (denominators).

Age appears as a long float and appears to be calculated in fractions that are 30/365 units apart. My reading of this is that the observation at 21.04109589 is the 21st birthday month.

age = age in month-fractional years
age21 = age recentered to be zero at age 21
allmort = mortality rate per 100,000, all causes
internal = mortality rate per 100,000, internal causes
external = mortality rate per 100,000, all external causes
alcohol = mortality rate per 100,000, alcohol-related causes (not motor vehicles)
homicide = mortality rate per 100,000, homicide
suicide = mortality rate per 100,000, suicide
mva = mortality rate per 100,000, motor vehicle accident
drugs = mortality rate per 100,000, drug related
otherexternal = mortality rate per 100,000, all other external cause